1. Field of the Invention
The present invention relates to a method of driving a liquid-crystal display device.
2. Description of the Related Art
A liquid-crystal display is generally a hold type of display that continues to hold display of the previous frame and not an impulse type of display as with CRTs (Cathode Ray Tubes). For this reason, image-blurring effects become a problem at the time of displaying moving image.
In observing a moving object on a display screen, the moving object image in a frame continues to be displayed in the same position until switching is made to the next frame. Nevertheless, human eyes will continuously follow the moving object. As a result, the image blurring effects occur. In other words, although the movement of the moving object displayed on the screen is discontinuous, human eyes will recognize the moving object in such a way as to interpolate between images of successive frames because of the continuity of their follow-up motion. This is the cause of the image blurring effects.
To solve the image blurring effects, a method has been proposed which divides one frame into an image display interval and a black display interval through the use of a fast response liquid crystal, such as an OCB (Optically Compensated Bend) mode liquid crystal or a ferroelectric liquid crystal.
One such method is field inversion driving (see, for example, Japanese Unexamined Patent Publication No. 2000-10076). In this field inversion driving, one frame is divided into two fields, and the liquid crystal layer is placed in the transparent state in the first field and in the nontransparent state in the second field. As the liquid crystal layer use is made of a liquid crystal that induces spontaneous polarization the magnitude of which varies with the polarity of applied voltage. That is, use is made of a liquid crystal in which the induced spontaneous polarization exhibits an asymmetric response when the polarity of applied voltage is changed. In this specification, a liquid crystal having such response characteristics is referred to as an asymmetric response liquid crystal. One such liquid crystal is a monostable ferroelectric liquid crystal, which has a fast response. To make a ferroelectric liquid crystal monostable, there are two methods: one to introduce a polymer network into the liquid crystal layer and one to subject the liquid crystal layer to an initial alignment process by cooling it slowly with a DC voltage applied thereto.
With the aforementioned liquid crystal display device, for example, in the first field the liquid crystal layer is driven with positive polarity for writing and in the second field it is driven with negative polarity for erasing (resetting). Thereby, AC driving is accomplished. In this case, the positive polarity is that to which the polarization responds (or the polarity to which the polarization responds to a larger degree). In other words, the positive polarity is that under which the change of light transmittance of the liquid crystal display device is greater. On the other hand, the negative polarity is that to which the polarization does not respond (or the polarity to which the polarization responds to a smaller degree). That is, the negative polarity is that under which the change of light transmittance of the liquid crystal display device is smaller.
In the case of the AC driving, if a DC component remains across the liquid crystal layer for a time much longer than one frame period, image sticking will occur. Thus, the driving should be performed so that no DC component will remain across the liquid crystal layer. Conventionally, it has been believed that the driving by voltages having the same amplitude but opposite polarity will eliminate the DC component and prevent the image sticking.
With the above driving method, the hold voltage between electrodes of each pixel becomes symmetrical. In practice, however, insulating films, including alignment films, are interposed between the electrodes and the liquid crystal layer. With a liquid crystal display device using an asymmetric response liquid crystal, the effective dielectric constant of the liquid crystal is asymmetric. Thus, the voltage divided into the liquid crystal layer and the insulating films becomes asymmetric. Thus, the DC component will remain across the liquid crystal layer. Experiments also confirmed that the imaging sticking occurred.
Thus, although a proposal has been made for an impulse type of display using an asymmetric response liquid crystal in order to prevent image blurring effects in moving image display, the conventional driving method presents a problem of the occurrence of image sticking due to application of a DC component across the liquid crystal layer.
An object of the present invention to provide a liquid crystal display driving method which permits the liquid crystal layer to be prevented from being impressed with a DC component.